Fuel cells for combining hydrogen and oxygen to produce electricity are well known. One known class of fuel cells is referred to as solid oxide fuel cells (SOFC's). An SOFC generally consists of a cathode and an anode physically separated by a solid oxide electrolyte, and electrically connected by an external electrical circuit. During operation of an SOFC, oxygen is provided to the cathode of the cell while hydrogen-containing fuel is provided to the anode. Oxygen is catalytically reduced at and diffuses through the cathode to reach the solid electrolyte. The solid electrolyte is permeable to the oxygen anions, which diffuse across the electrolyte to the anode where they combine with hydrogen to form water and release electrons, which flow through the external circuit to the cathode to provide the source of electrons for the catalytic reduction of oxygen, thereby generating electricity.
The cathode of an SOFC must satisfy a combination of criteria, including but not limited to catalytic activity, electrical conductivity (both ionic and electronic conductivity), electronic structure, stability, structural integrity, thermal expansion, and the like, all at operating conditions such as high temperatures in excess of 800° C. A class of materials that have used for SOFC cathode applications is the mixed ionic and electronic conducting (MIEC) materials such as perovskite-type ABO3 oxides. The general chemical formula for perovskite compounds is ABX3, wherein ‘A’ and ‘B’ are two cations of very different sizes, and X is an anion that bonds to both. The native titanium mineral perovskite itself is of the formula CaTiO3. The ‘A’ atoms are larger than the ‘B’ atoms. The ideal cubic-symmetry structure has the B cation in 6-fold coordination, surrounded by an octahedron of anions, and the A cation in 12-fold cuboctahedral coordination. The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations, or both are reduced.
Mixed oxide materials comprising lanthanum (La), strontium (Sr), cobalt (Co), and iron (Fe), also known as “LSCF”, of have been proposed in the prior art as materials for SOFC cathodes due to their high catalytic activity for the oxygen exchange reaction and a high electronic conductivity for current collection. One proposed formulation is characterized by the general formula La1−xSrxCo1−yFeyO3−δ. The physical and chemical properties of this class of materials, such as electrical conductivity, electronic structure, catalytic activity, stability, and thermal expansion coefficient (“TEC”), have been studied in detail. Generally, electronic and ionic conductivities and catalytic activity are enhanced with increasing values of x and decreasing values of y, whereas there is an opposite tendency for chemical stability.
Although many of the materials proposed for SOFC cathodes have been effective to varying degrees, alternative materials that offer better performance, reliability, cost, or combination of these or other parameters.